Ultrasound and X-ray are powerful imaging technologies that have found widespread use in clinical settings and in other applications such as inspection and examination of materials. Each technology has its strengths in producing images of different types of materials, and accordingly the use of either technology for a given application is often determined on this basis. However, in applications involving diverse materials it may be advantageous to use both techniques, so to acquire images of diverse features of interest. For example, certain medical procedures, such as brachytherapy, would benefit from the combined use of ultrasound and X-ray imaging.
Brachytherapy involves the placement of radioactive pellets or “seeds” into or adjacent cancerous tissue of a patient. Brachytherapy makes it possible to treat the cancer with a high total dose of radiation in a concentrated area in a short period of time, and at the same time spare healthy tissues the treatment with radiation. The key to successful brachytherapy is the accurate placement of the seeds. However, faulty needle and seed placement often cause an insufficient dose to the cancer and/or inadvertent radiation of healthy tissues. The ability to perform dosimetry optimization during the procedure could change the standard of care in brachytherapy, but such function is not available today and it is unfortunate that implants are currently performed without an explicit dosimetry evaluation in the operating room. Generally, dosimetric analysis requires precise localization of the implanted seeds in relation to the cancerous tissue and surrounding anatomy.
Brachytherapy has become the predominant treatment option for early stage prostate cancer. The procedure entails permanent implantation of radioactive seeds into the prostate to eradicate the cancer with ionizing radiation. In the case of prostate cancer, inadvertent radiation of the rectum, urethra or bladder through incorrect seed placement may result in adverse side effects such as rectal ulceration, incontinence, and painful urination.
Prostate brachytherapy is typically performed with transrectal ultrasound (TRUS) guidance that provides adequate real-time visualization of the prostate but not of the implanted seeds. Despite significant efforts, localization of seeds directly from TRUS has not been clinically practical or robust. C-arm fluoroscopy is often used for gross visual assessment of the implant but it cannot show the prostate. Recently, accurate reconstruction of seeds from fluoroscopy has become possible [2-5].
In specially-equipped operating rooms, computed tomography (CT) imaging [6] or cone beam CT [7] imaging is available. Because the shortcomings of TRUS and X-ray offset each other, quantitative dosimetry could be performed with spatial registration of the two. In order to register TRUS and fluoroscopy, Zhang et al. [8] suggested affixing radio-opaque fiducials to the TRUS probe, thereby permanently altering standard clinical equipment. Jain et al. [2] proposed precision machined fiducial structure calibrated to the needle guide template. Gong et al. [9] used needle tips as fiducials for the registration. For a variety of reasons specific to the brachytherapy workflow, fiducial-based approaches are not sufficiently reliable or clinically practical. Su et al. [3] and Tutar et al. [4] suggested point-based registration between seeds segmented and reconstructed in both fluoroscopy and TRUS. The generally poor quality of TRUS (due to noise, speckle, acoustic decoupling, calcifications masquerading as seeds, shadowing, multiple reflections, etc.) makes seed segmentation prone to error, causing instable registration performance.